Java JDBC query in separate thread lock parent

Question

I'm having some trouble understanding this. Here's what's happening. I'm spawning a new thread that holds a JDBC connection to a Oracle database. When I ask it to connect to the database, the parent thread keeps running while the start() method is invoked but when I ask the child to execute a query (on a separate method), the parent thread gets stuck waiting for the child thread's method to finish doing its work. Any guess as how to fix this? Thanks in advance! ``` public class Main extends Thread{ public Main() { } public void myCounter() { int i = 0; DBConnection myConnection = null; for(;;) { i++; System.out.println("time: " + i); if( i == 5) { myConnection = new DBConnection("localhost", 1521, "hr", "hr", "XE"); myConnection.start(); } if(i == 10) try { myConnection.runQuery("Select * from hr.numbers order by dbms_random.value"); } catch (SQLException e) { e.printStackTrace(); } try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } public void run…

Accepted Answer

I think you have misunderstood a bit how threads work. The problem is that you are calling this on the parent thread:

```
myConnection.runQuery("Select * from hr.numbers order by dbms_random.value");
```

which is a sequential invocation of the `runQuery` method on the `myConnection` object that happens to be a thread. This does not mean that it will instruct the child to execute the method. Instead the parent will execute it itself and the child thread finishes as soon as its `run` method has returned.

If you want to have a separate thread that keeps receiving commands to execute queries you will have to implement a producer-consumer pattern, where the parent keeps queuing commands for the child to execute. To this end I recommend you look at ExecutorService.

Java JDBC query in separate thread lock parent

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